The present invention relates generally to equipment utilized, and operations performed, in conjunction with subterranean wells and, in embodiments described herein, more particularly to a two slip retrievable packer for extreme duty.
Conventional hydraulically set retrievable packers have three main components, at least one slip, packing elements and a setting cylinder all of which are assembled on a mandrel. Typical packers have a dual slip on top, packing elements in the middle and a setting cylinder on the bottom. This design works well in many typical well applications. However, the design has limitations on the loads it can bear. Upthrust on the mandrel due to pressure below the packer and/or applied upstrain on the tubing above sometimes cause excessive loads to be generated in the packing element. The packing element sustains two additive loads in this design. One load is the applied hydraulic pressure differential across the packer. This pressure is contained by the well casing and the element mandrel on which the packing elements ride. Any upthrust on the mandrel eventually terminates at the dual slip and into the casing. This second load must be transmitted through the packing elements to reach the slip. This mechanical load translates into additional element pressure, i.e. rubber pressure. This pressure is additive to the imposed hydraulic pressure.
For example, an extreme packer application may call for a packer to withstand 10,000 p.s.i. differential hydraulic pressure imposed from below, plus an additional tubing tension load, or upward pressure differential load on the mandrel, of 300,000 pounds. The mechanical load adds to the rubber pressure from the hydraulic pressure load. The rubber area exposed to the mechanical load is the difference in area of the casing internal diameter and the element mandrel outer diameter. In a typical case this area may be 25 square inches. For this area, a 300,000 pound load creates 12,000 p.s.i. rubber pressure, i.e. 300,000 divided by 25. This mechanically generated pressure load adds to the actual hydraulic pressure to produce a total pressure load on the packing elements of 22,000 p.s.i. The casing is subjected to this pressure as a burst load, and the mandrel is exposed to this pressure as a collapse load. In many cases, the well casing cannot be expected to sustain this pressure. If it does not have a solid cement sheath, it will fail.
In U.S. Pat. No. 6,112,811, a packer having two dual or double acting slips provided a solution to the problem of combined mechanical and hydraulic pressure loads on the packing elements. In that system, two dual slips were arranged so that one half of each dual slip resisted hydraulic pressure, up or down, applied to the packing elements and the other half of each dual slip resisted loads applied to the mandrel. The packing element was not exposed to a combination of the two types of loads. In that arrangement, the well casing is used as a tension member to store at least part of the setting force of the packer. This is a typical arrangement in two slip permanent packers also. However, in order for the packer to be retrievable, there must be some mechanism for effectively shortening the mandrel between the slips to release the tension in the well casing so that the dual slips can release from the casing. As reference to U.S. Pat. No. 6,112,811 shows, such releasing mechanisms require multiple releasing elements for releasing the setting force on the packing elements and for applying releasing forces to multiple wedges in order to actually release the two dual slips. Failure of one or more of the releasing elements to function properly may prevent retrieval of the packer or may require use of an explosive tubing cutter to sever the mandrel. It may be necessary to destroy the packer in order to remove it from the well.
It would be desirable to provide a packer which avoids excessive forces applied to packing elements, and which has a simplified releasing mechanism.